Method of manufacturing microfine metal powder



Feb; 10, 1970 I WATARU :smaAsm METEOD OF MANUFACTURING MICROFINE METAL POWDER Filed Nov. 27,1967

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METHOD OF MANUFACTURING MICROFINE METAL POWDER Filed Nov. 27, 1967 2 Sheets-Sheet 2 INVENTOR wot'aru lsmmm AORNEY States 23 Claims ABSTRACT OF THE DISCLOSURE A method of producing microfine powder of metal, alloy or other electroconductive substances in which spark discharges are caused among the electroconductive material grains or pellets and by the resultant electrolytic corrosion the microfine powder is obtained. Electrically insulating liquid and the material grains or pellets are placed in a reactor of horizontally lying cylinder or prism type with a pair of electrodes to be charged with current disposed on the opposite sides in the reactor. By bringing the reactor into rotation on the axis of its main body the grains of metal, alloy or other electroconductive sub stances are forcibly agitated and the spark discharges are thereby more vigorously eflected.

CROSS-REFERENCES TO RELATED APPLICATIONS The present application is a continuation-in-part of my copending application Ser. No. 456,645, filed on May 18, 1965, and now US. Patent No. 3,355,279, issued on Nov. 28, 1967, entitled Method of Manufacturing Microfine Metal Powder.

BACKGROUND OF THE INVENTION It has conventionally been known to obtain fine metal particles less than 0.1 millimeter in average diameter by mechanical means such as grinding a metal directly, crushing it under high impact pressure, or spattering a molten mass of a metal, or by an electrical means through which a desired metallic compound is electrolyzed and the resulting metal deposit on the electrode of an electrolytic cell is pulsified in a suitable manner. Such known methods, however, have substantial and inevitable disadvantages as already described in the parent Patent N0. 3,355,279. The inventor accomplished an invention, application Ser. No. 456,645 filed on May 18, 1965, and issued as Patent No. 3,355,279 on Nov. 28, 1967, wherein the above disadvantages are eliminated and an outstanding method is presented for producing microfine powder of metal or alloy which is uniform in size and yet about 0.05 millimicron in diameter. The invention relates to a method of producing microfine metal powder by making use of spark discharges which take place between metal electrodes, wherein material metal grains are placed between a pair of electrodes provided in electrically insulating liquid medium, said grains being disposed so as to serve as an intermediate electrode between the electrodes.

When the material metal grains serving as intermediate electrode are acted on by the pulsating current injected between the electrodes, spark discharges take place therein, so that, by the resultant discharge impact pressure and the expansion of the gas generated upon the instantaneous thermal cracking of the liquid medium among metal grains at the sparking points, the material metal grains are agitated and fluidized, while by the spark energy fragments are electrolytically scraped ofl the material grains whereby microfine powder of the metal or alloy is produced. In producing metal powder according to this method, it

3,494,762 Patented Feb. 10, 1970 is essential that, in addition to the stabilization of the discharges, the discharges should take place at ever-shifting points with high frequency. If, for example, the spark discharges take place while the metal grains are left in static condition and not agitated and fluidized, difficulties arise that the discharges effected only at limited location cause the fusion between or among the metal grains thereby resulting in short-circuits and unstable discharges and the ability to yield powder is extremely deteriorated in consequence. In the primary application, the inventor stated in the claim that the insulating liquid is forcibly circulated upward from the bottom of the reactor, whereupon, by the force of the fluid, the material metal grains therein are further agitated and fluidized with the result that the vigorous spark discharges are eflectively provided among each of the grains. Relating to the above described method of production, the inventor now presents another method capable of obtaining increased yield of the metal powder in which is employed a device for effecting discharge reaction simple in structure and less susceptible to troubles.

SUMMARY OF THE INVENTION As above-mentioned, the original invention provides a method of production which comprises charging a reactor having a pair of electrodes and an electrically insulating liquid therein material grains or pellets of metal, alloy or other electroconductive substances, and injecting pulsating current between said electrodes to produce spark discharges among said material grains put in the insulating liquid whereby microfine powder is produced from the electrolytically corroded grains of said material, in which method it is contemplated in the principal claim that the grains or pellets in the insulating material are agitated and fluidized by such means as the impact pressure of the spark discharges and expansion of the insulating liquid due to the cracking decomposition.

It is further stated in another claim that the insulating liquid is forcibl circulated upwardly from the bottom in the reactor of an upright type to get the grains better fluidized and agitated. However, in order to obtain much more effective spark discharges among the grains or pellets and to get them to take place repeatedly at ever-shifting points, the inventor herein presents an invention stated below.

The present invention is characterized in that a reactor has a horizontally lying and elongated cylinder body which is set in rotation with a pair of electrodes, an insulating liquid and material grains or pellets provided therein. According to the invention, excellent agitation and fluidization are effected to the material metal grains put in the insulating liquid within the reactor due to the rotation of the reactor itself.

One of the most distinguished objects of the present invention is to provide the metal grains to be comminuted into microfine particles with more active agitation and fluidization than in the original inventon, regardless of the specific gravity of the metal grains, whether they are regular or irregular in shape, and even when the grains are relatively varied in size and they are comparatively large masses, whereby the discharges can take place at ever-shifting sparking points in uniform and smooth action thus securing eflicient production of microfine powder.

Other objects and features of the present invention will become apparent with reference to the accompanying drawings, from the below stated embodiment involving the details of:

Shape, structure, dimensions, means of rotation and number of revolutions of the reactor;

Position, shape, structure of a pair of electrodes mounted in the reactor, spacing between the electrodes and means of inducing current therebetween;

Structure and characteristics of pulsating current generator provided between the electrodes;

Property of pulsating current between the electrodes;

Type and quality of insulating liquid provided in the reactor;

Type, size and quality of material metal, alloy or other electroconductive substances to be supplied to the reactor; and

Quality and properties of microfine metal powder produced.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a block diagram showing the disposition of devices to be employed for carrying out the method of the present invention;

FIG. 2 is a schematic diagram in cross section showing the most fundamental of manufacturing equipment employed in the primary invention;

FIG. 3(a), a front view, and FIG. 3(1)), a side elevation are schematic diagrams showing the most typical example of the equipment to be employed in carrying out the present invention;

FIGS. 4 to 7 are views in cross section showing the interior of different examples of reactors to be employed in accordance with the present invention; and

FIG. 8 is a diagram showing a circuit of discharge voltage generator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 in which disposition of the elements of the present invention is shown, there is illustrated centrally a reactor in the form of a horizontally lying cylinder, with a pair of electrodes mounted within, or at the opposite ends of, the cylinder and between the electrodes is disposed a discharge voltage generator. The discharge voltage generator comprises a group of spark discharge gaps connected in series to the electrodes and an auxiliary coil connected in parallel to the electrodes. The reactor of a horizontal cylindrical type is provided with a means to bring it in rotation on the axis thereof, the number of which revolutions is variable. The opposite parallel sidewalls of the reactor are provided with circular openings concentric with the axis of the rotation, one of said openings serving as a material inlet, the other an outlet for the product. At the material inlet is located a pump for feeding material and further ahead there is a material supply device with a measuring means interposed therebetween. The outlet of the reactor is provided with a separator to separate the metal powder from the liquid medium, through which the former is carried into a selector of metal powder and the latter, together with the power inadequate to the requirements of product, is flowed back to the material supply device and is recirculated. The most important of the present invention among the devices in the above-mentioned disposition are the reactor in the form of a horizontal cylinder involving electrodes, means of rotation therefor and a spark voltage generator placed between the electrodes. The method in accordance with the present invention for producing microfine metal powder will be described further in detail with reference to the most typical embodiment shown in FIG. 3. The reactor 1 comprises a main body 2 in the shape of horizontally lying hollow cylinder made of an electrically insulating substance and opposite parallel side walls serving as a pair of electrodes 3 and 4 facing towards each other, formed in fiangeshape and made of an electroconductive substance such as iron, copper and the like. The reactor is commonly circular in section, while it may be formed of a prismatic body having a regular polygonal section shown in FIG. 7.

The Opposite side walls themselves of the reactor serve as a pair of electrodes 3 and 4 as above described, while a reac or entirely constructed of an electric l y insulatin substance in horizontal hollow body may be provided, for instance, with doughnut-shaped electrodes 3 and 4 which are disposed within the reactor a suitable distance apart in facing relation.

In the opposite side walls of the reactor 1, whether or not said walls are employed as electrodes 3 and 4, relatively large circular openings 5 and 6 are formed concentrically therewith. The diameter of the circular openings 5 and 6 is seldom, if ever, larger than half the length of the diameter of the section of the reactor 1.

The exterior peripheral surface of the main body 2 of the reactor is provided with a pair of guide rails 7 and 8 mounted on and fitted to the two pairs of driving rollers 9, 10, 11 and 12 for elfecting rotation of the reactor which are positioned beneath the main body, so that the driving rollers 9 and 11, upon being set rotating by means of a motor 13, a speed changing gear 14 and a speed reducing device separately fixed in place, bring the entire body of the reactor 1 into rotation. The electrodes 3 and 4 in the opposite side walls of the reactor 1 having the aforementioned circular openings 5 and 6 are provided with brushes 16 and 17 which are adapted to abut and press against each surface of the electrodes, each of said brushes being connected, by means of conductors 18 and 19, to a discharge voltage generator 20 positioned apart from the reactor 1. Shown in FIG. 8 is the discharge voltage generator 20, which in the illustrated embodiment is a quenched spark gap oscillator adapted to yield a large number of effective spark discharges per unit time under a high discharge energy. The oscillator has discharge gaps G, which are connected in series to the electrodes 3 and 4. Also in the quenched spark gap oscillator 20, 21 and 22 are contacts leading to a power supply, 23 is a step-up transformer, 24 is a limiting reactor, 25 is a charge-discharge capacitor, 26 is a group of spark discharge gaps G, 27 is an oscillating inductance, 28 is an inductance disposed in parallel to the electrodes 3 and 4 so as to make the spark discharges more effective, 29 and 30 are chokecoils, and 31 and 32 are non-dielectric resistors. The above-mentioned discharge voltage generator 20 is capable of generating a pulsating current of 10 to 30 kilovolts and 20 to kilocycles per second. The method for producing microfine powder of metal, alloy or the like in which the principal devices mentioned above are employed will be now described. Grains, pellets and small pieces A of electroconductive metal or alloy having an average particle diameter of 3 to millimeters are fed into the reactor 1 by means of the material supply device (not shown in the figure) and then an insulating liquid B is put in the reactor 1 through the circular opening 6 in the side wall 4 on the inlet side thereof. In this particular embodiment of the present invention, aluminum pellets having a diameter of 10 millimeters are used for material metal, and fuel oil for an insulating liquid. Upon operation of a pump (not shown in the figure) in the material supply device, the fuel oil within the reactor 1 of horizontal cylinder type maintains its liquid level at the edge of the center circular openings 5 and 6 in the electrodes 3 and 4, the excess oil overflowing out of said circular opening 5 in the outlet side wall. Under this condition when the reactor 1 is driven by the rotation of the driving rollers 9 and 11 while pulsating voltage being simultaneously injected by the discharge voltage generator 20 across the electrodes 3 and 4, spark discharges occur among the material pellets of aluminum with the result that each of the pellets are spattered by the discharge impact while part of fuel oil electrolytically decomposed and liberated bubbles, the formation of bubbles further causing contact and repellency among the grains whereby the spark discharges become more active.

Furthermore according to the present invention, because the reactor itself is set in rotation, the fuel oil therein is agitated, the material aluminum pellets in the fuel oil also being forcibly agitated and fluidized so that the pellets which normally tend to deposit in the lower portion of the fuel oil by reason of specific gravity now take an effective part in the reaction. In this way, the most adequate and effective spark discharges caused among the pellets scrape microfine metal particles off the pellet surfaces in electrolytic corrosion, which particles are then suspended in the fuel oil and overfiowed from the outlet in the reactor thereafter being separated from the fuel oil by a separator (not shown in the figure). From among the rnicrofine particles thus obtained those having a desired average diameter are selected and taken out as products. On the other hand, the fuel and the rnicrofine powder with particle sizes inadequate to the requirements are flowed back to the material supply device by a pump.

The following is detailed numerical values as to the above embodiment:

Dimensions of reactor 500 mm X 800 Material of reactor Polypropylene.

Material of electrodes Aluminium.

Size and shape of electrodes Circular plate of 500 mm center opening 250 mm. in diameter. Spacing of electrodes 800 mm. (length of the reactor). Speed of rotation of reactor 60 r.p.m. Source voltage of discharge voltage generator 220 v. Source frequency of discharge voltage generator 60 c./s. Discharge voltage ZOkV. Frequency of discharge current 50 kc./s. Frequency of discharges 6 per /2 cycle. Amount of fuel oil in reactor 50 L./hr. Amount of Al pellets 12 kg. Average size of Al pellets mm. Average size of microfilm A1 particles (product) 0.3 micron. Yield of microfilm Al powder 5 kg./hr. Power consumption 22 kWh.

Besides the above described embodiment, according to another embodiment in which the inside wall of the main body of the reactor 1 is, as shown in FIG. 4, provided with partition walls 33 disposed in axial direction and peripherally equidistantly, further excellent agitation and fluidization of the grains are achieved permitting the discharges to occur in three-dimensional extension with increased frequency, so that a large amount of rnicrofine metal powder can be produced with efficiency.

Furthermore, the present invention is carried out much more effectively by an example shown in FIG. 5 which has fins 34 attached to the ends of the above-mentioned partition walls 33 in FIG. 4 and adapted to move back and forth in the peripheral direction. An example shown in FIG. 6 also achieves remarkable results in executing the present invention in which example the width in radial direction of each partition wall 33 is gradually increased towards the direction of rotation.

The above embodiments, as aforesaid, are only one mode of example embodying the principles of this invention. In another mode of embodiment, the present invention employs water in place of an insulating liquid such as heavy oil. The advantage of using water obviously lies in its low cost. It is to be understood, however that there arises a marked difference in results between these alternatives. For when water is used as the reaction medium, the resulting rnicrofine metal or alloy particles are oxidized and hydroxidized. Therefore, this method is excellent as a method to obtain oxidized and hydroxidized rnicrofine powder.

In still another mode of embodiment, a liquefied inert gas such as liquid nitrogen or liquid argon is employed in place of an insulating liquid. When such a liquefied inert gas is put to use, the reactor must be provided with a suitable heat insulating structure. An advantage of this method is that the rnicrofine powder of metal or alloy produced, when left intact in the air, can be directly and readily taken out without the use of a particular separator and that the metal powder obtained is free from oxidation. In addition, there is also an advantage that if the inert gas evaporated is caught and compressed, it can be put to use once again. Unlike a case where liquid hydrocarbons are employed as a reaction medium, this method in which no carbon is produced by cracking saves the necessity of separating such a by-product from the product. This feature more than makes up for the additional cost involved in the provision of the heat insulation. Another feature of the present invention is that since the rotating reactor gets the insulating liquid and grains forcibly agitated, making, in particular, the grain fluidized and agitated, the spark discharge is satisfactorily effected even if the insulating liquid should have a relatively high viscosity. Accordingly, the present invention is useful especially in the case where rnicrofine magnetic iron powder is produced in the mixture with a colloid or highly viscous natural resin or synthetic resin such as an epoxy resin before being mixed with a hardening agent. While grains can 'be used as material regardless of the difference in specific gravity, size, shape and the like as previously mentioned, the material in addition is not necessarily restricted to the electroconductive substance but a semiconductive substance such as silicon, metal with sintered ceramic or the like, can also be employed. It is expected that the present invention has a very promising future in this field of utilization.

As apparent from the foregoing description, the present invention provides a method for production wherein, by rotating the reactor of a vertical cylinder type, material grains or pellets of any metal, alloy or any other electroconductive substance can be more effectively and adequately agitated and fluidized without ever being influenced by any restrictive conditions, whereby rnicrofine powder of the same quality as in the original invention can be produced.

What I claim is:

1. A method of manufacturing rnicrofine metal powder which comprises the steps of: preparing a mixture of an electrically insulating liquid with electroconductive grains of metals, and alloys subjecting said mixture to a pulsating current between a pair of spaced electrodes, thereby causing spark discharges among said grains until electrolytically corroded rnicrofine particles of said grains are obtained, placing said mixture into a reactor having a horizontally lying elongated main body made of an electrically insulating substance, with openings formed centrally in the opposite side walls of said reactor, one of said openings serving as an inlet for said liquid, the other of said openings serving as an outlet, said pair of electrodes being disposed in said reactor in facing relation rotating said mixture on the axis of the main body, and connecting said electrodes to a discharge voltage generator until said grains are forcibly agitated and fluidized regardless of the difference in specific gravity, size and regularity or irregularity of shape so as to make the spark discharges more vigorous.

2. A method as claimed in claim 1, said liquid being an insulating liquid.

3. A method substantially as claimed in claim 1, said insulating liquid being water whereby oxidized metal rnicrofine powder and hydrolyzed powder are produced from said electroconductive substance.

4. A method as claimed in claim 1, wherein said liquid is selected from the group consisting of petroleum type liquid hydrocarbons and the liquid hydrocarbons of the animal and of the vegetable oil type.

5. A method as claimed in claim 1, wherein said liquid is selected from the group consisting of liquid nitrogen, liquefied argon, and other liquefied inert gases.

6. A method as claimed in claim 1, wherein said insu lating liquid is selected from the group consisting of highly viscous natural resins, synthetic resins, colloid natural resin and phenolic epoxy resins.

7. A method as claimed in claim 1, wherein said grains comprise semiconductive substances selected from the group consisting of silicon and metal with sintered ceramic.

8. A method as claimed in claim 1, wherein an average diameter of said material grains is selected from the range of 3 to 100 millimeters.

9. A method as claimed in claim 1, further comprising the steps of maintaining a fixed level of said liquid and said simultaneously circulating said liquid.

10. A method as claimed in claim 1, wherein the speed of rotation of said reactor is variably selected from the range of 5 to 100 rpm.

11. A method as claimed in claim 1, wherein said discharge voltage between said electrodes is selected from the range of to 30 kilovolts.

12. A method as claimed in claim 1, wherein the frequency of discharge current is selected from the range of 20 to 80 kilocycles per second.

13. An apparatus for manufacturing microfine metal powder, comprising an elongated reactor made of an electrically insulating material with an inlet and outlet opening respectively in each end thereof, in alignment with the longitudinal axis of said body; means to rotate said reactor along its elongated axis, said axis mounted in the horizontal position; a pair of electrodes mounted spaced from each other in facing relationship within said reactor; a source of current and means to connect said electrodes with said source of current to a discharge voltage, while simultaneously rotating said reactor along the horizontal axis thereof to agitate and fluidize a mixture of a fluid with electroconductive grains placed in said reactor regardless of their difference in specific gravities, sizes and shapes, whereby the vigor of the spark discharge is increased.

14. An apparatus as claimed in claim 13, wherein the main body of said reactor has a circular cross section.

15. An apparatus as claimed in claim 13, wherein the main body of said reactor has a regular polygonal section.

16. An apparatus for carrying out amethod as claimed in claim 13, wherein the outer peripheral surfaces of said reactor are provided with guide rails near to the opposite parallel sides thereof, two pairs of driving rollers for effecting rotation being fitted to said rails to carry said reactor thereon.

17. An apparatus as claimed in claim 13, wherein the wall of said reactor is interiorly provided with partition walls which are disposed in axial direction peripherally equidistantly spaced.

18. An apparatus as claimed in claim 17, wherein an end of each of said partition walls in the reactor is further provided with a fin which is adapted to lean back and forth in the peripheral direction.

19. An apparatus as claimed in claim 17, wherein the width in radial direction of each of said partition walls within the reactor is gradually increased towards the direction of the rotation of said reactor.

20. An apparatus as claimed in claim 13, wherein said reactor is to 1000 millimeters in length.

21. An apparatus as claimed in claim 13, wherein said reactor is constructed of an electrically insulating resin material.

22. An apparatus as claimed in claim 13, wherein a discharge voltage generator to be disposed between said electrodes is provided with spark discharge gaps which are connected in series to said electrodes.

23. An apparatus as claimed in claim 13, wherein a discharge voltage generator to be disposed between said electrodes is provided with an auxiliary coil connected in parallel to said electrodes.

References Cited UNITED STATES PATENTS 3,282,814 11/1966 Berghaus 204164 3,355,279 11/1967 Ishibashi -0.5

L. DEWAYNE RUTLEDGE, Primary Examiner T. R. FRYE, Assistant Examiner US. Cl. X.R. 

